The present invention relates to a single-fluid lithographic printing process and to method and means for carrying out that process.
In the art and practice of high-speed lithographic offset printing, ink is more-or-less continuously conveyed from a suitable reservoir by means of a series of coextensive rollers to a planographic printing plate where the image portions of the printing plate accept ink from the last of the series of inking rollers. A portion of that ink is then transferred to a printing blanket as a reverse image from which a portion of the ink is transferred in the form of a right-reading image to paper or another suitable substrate. It is essential in conventional lithographic printing processes that dampening water containing proprietary additives also be conveyed more-or-less continuously to the printing plate where, by transferring in part to the non-image areas of the printing plate, the water operates to keep those non-image areas free of ink.
In conventional printing press systems, both the ink and the dampening water are continuously and separately made available to all parts of the printing plate, image and non-image areas alike: and in the absence of dampening water, the printing plate will accept ink in both the image and non-image areas of its surface.
Lithographic printing plate surfaces in the absence of imaging materials have minute interstices and an overall hydrophilic or water-loving character that enhance retention of water rather than ink in the non-imaged areas. Imaging this hydrophilic plate surface creates oleophilic areas according to the desired image format. Subsequently, when water is presented to the inked, imaged plate in appropriate amounts only that ink residing in non-image areas becomes debonded. In its simplest view, this action accounts for the continuous image and non-image differentiation at the printing plate surface which differentiation is essential and integral to the lithographic printing process.
Controlling for the correct amount of dampening water input during lithographic printing has been an industry-wide problem ever since the advent of lithography. Doing so requires continual operator attention since each change in ink input appears to require a change in dampener input. However, balancing the ink input across the width of the press with dampener input across the width of the press is at best a compromise. Consequently, depending upon which portion of the image format the operator has selected for complying to a standard print quality at any given time during the printing run, he may need to adjust the ink input at that cross-press position which inadvertently also changes the water balance at that position. Conversely, the operator may adjust the dampener input to balance one portion of the image. This action may affect the ink and water balance at one or more other cross-press locations. Adjustments of these types tend to occur repeatedly throughout the whole press run resulting in slight to major differences in the quality of the printed output throughout the run. In carrying out these adjustment operations, the resulting copies may or may not be commercially acceptable, leading to waste in manpower, materials, and printing machine time.
Means for correcting this inherent fault of conventional lithography have been addressed; none have achieved industry-wide success. These methods involve either eliminating the dampening system or they involve eliminating operator control of the dampening system.
Newspaper printing configurations are known which rely on the inking train of rollers to carry dampening water to the printing plate. Reasons why the ink-train-dampening system operates especially well without alcohol or similar dampener additives as traditionally specified for use with other existing ink-train-dampeners are not clear. Configurations such as those noted above will, together with appropriate ink and dampening concentrate selections, function such that the ink itself carries all of the required dampening water to the printing plate, yet the press functions and is controlled more-or-less conventionally from the viewpoint of lithographic printing. Accordingly, all of the conventional problems inherent in attaining and maintaining the optimum balance of ink and water input also attend this type of ink-train-dampening lithographic printing press system.
Planographic printing systems and elements thereof which do not require dampening water, and may therefore be termed single-fluid systems, are known in the prior art. Such systems rely in one way or another on low-surface-energy silicone non-image portions of the printing plate disallowing ink adhesion, thereby forming the basis for differentiation between ink-receptive nonsilicone image areas and of non-ink-receptive non-image silicone areas of the printing plate. Only ink needs to be available to the plate, dampening solutions being unnecessary. These single-fluid planographic printing systems enjoy limited commercial success because of higher plate cost and because the more-expensive printing plates have a relatively short useful lifetime on-press. In addition, the ink needs to be formulated to take into account that the press temperature varies during printing causing the ink transfer efficiency to change, necessitating more-or-less continual operator adjustment of inking keys. Thus, although no dampening fluid adjustments are necessary, cooling of press cylinders may be required, as well as considerable operator attention to compensate for gradual heat-induced inking changes which render these systems of limited commercial value. For these reasons usefulness of these systems has in practice been limited to short runs, generally of 50,000 copies or less.